The Excretory System PDF

The Excretory System (focusing on Urinary System) Homeostasis  Excretion is the process of separating waste from body fluids & removing them from the body. The excretory system removes most metabolic wastes and maintains the...

The Excretory System (focusing on Urinary System) Homeostasis  Excretion is the process of separating waste from body fluids & removing them from the body. The excretory system removes most metabolic wastes and maintains the composition of the blood & body fluids. The waste is eliminated as urine and the system plays a major role in homeostasis. The removal of solid food waste or faeces is not a part of excretion since this material never enters the body. The kidneys are the organs responsible for processing fluids & the body can only survive a few days if they fail completely. Excretory System & Kidney Structure  Protein metabolism produces nitrogenous waste - toxic ammonia. The ammonia is converted to urea by the liver (which is only slightly less toxic) & the kidneys filter the urea from the blood and remove it with water as urine. This also maintains the water-salt balance of the blood. The colour of urine is an indicator of a person’s level of hydration. Nephrons - The Filtration System The filtering process takes place in the numerous nephrons of the kidney. The nephron is the functional unit of the kidney and reaches from the cortex through to the inner adrenal medulla. The kidneys filter between 1600 - 2000 mL of blood per day. Approximately 1.8 L of urine is produced. Structure of the Nephron Glomerulus - Network of capillaries where water and dissolved solutes are removed from the blood into the bowman’s capsule Bowman’s Capsule - Sac surrounding the glomerulus that collects water and solutes from the blood, producing filtrate Loop of Henle - Long, u-shaped portion of the tubule for the reabsorption of essential solutes back into the blood and additional removal of harmful substances (such as urea) Collecting Duct - Tubule where many other nephrons connect; promotes additional water reabsorption before urine is sent to the bladder for excretion Filtration in the Nephron The first step is to squeeze as much of the water & small solutes from the blood into the nephron through glomerular filtration. This filtrate will then move through the tubule and most of the materials will be reabsorbed into the blood. The water moves via osmosis as it follows the materials that are actively transported into the kidney tissue or blood. Step 1 - Glomerular Filtration Blood enters the glomerulus through the afferent (incoming) arteriole High osmotic and hydrostatic pressure forces water out of the capillaries through a filter, composed of multiple cell layers. This filter allows the movement of water and small solutes (such as ions, urea, amino acids, and glucose) but prevents large substances (such as red blood cells and proteins) from leaving the arteriole. The substance leaving the glomerulus is called filtrate, which is collected by the Bowman’s capsule and moved into the proximal tubule. Step 2 - Tubular Reabsorption and Secretion As the filtrate moves through the tubule, essential solutes and water are reabsorbed into the blood, and additional waste is secreted from the blood into the tubule. An overview of where and what type of transport is occuring in the nephron is shown in the diagram. The movement of ions alters the solute concentration of the medulla, pulling the water along via osmosis. Step 3 - Water Reabsorption The filtrate enters the collecting duct, where multiple nephrons connect As filtrate moves down this tube, the concentration of ions outside the duct gets higher. This further drives water out of the collecting duct via osmosis, promoting additional water reabsorption and creating concentrated urine. Urine is then sent to the bladder via the ureter. *This video did not mention bicarbonate, which is important. (See video on Slide 12) Women = XX at birth Men = XY at birth Homeostasis and the Kidneys  The kidneys other major function is to maintain the water-salt balance of the body fluids by adjusting the amount of water absorbed. There are osmoreceptors in the blood vessels that detect osmotic pressure which increases with blood volume. This will signal the hypothalamus to release ADH (antidiuretic hormone) which increases the permeability of water in the distal tubule & collecting duct. Certain foods, like caffeine or alcohol, block the release of ADH which increases urine output & dehydrates the body.  Aldosterone is released from the adrenal glands in response to maintaining blood pressure. In response to low blood pressure, it causes the bloodstream to reabsorb water with the sodium ions to increase blood volume. In response to high blood pressure, it causes the bloodstream to reabsorb less water as fewer sodium ions are reabsorbed to decrease blood volume.  The kidneys also assist the respiratory system in maintaining blood pH by supporting the carbonic acid/bicarbonate buffer system.